1.查询是否安装yum
rpm -qa |grep yum
2.没有安装，分别执行以下命令：
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-urlgrabber-3.10-10.el7.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-cron-3.4.3-168.el7.centos.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-3.4.3-168.el7.centos.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-54.el7_8.noarch.rpm
rpm -ivh --force --nodeps python-urlgrabber-3.10-10.el7.noarch.rpm
rpm -ivh --force --nodeps yum-cron-3.4.3-168.el7.centos.noarch.rpm
rpm -ivh --force --nodeps yum-3.4.3-168.el7.centos.noarch.rpm
rpm -ivh --force --nodeps yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
rpm -ivh --force --nodeps yum-plugin-fastestmirror-1.1.31-54.el7_8.noarch.rpm